Field of the Invention
This invention relates to marine seismic systems and more particularly relates to the design of seismic nodes deployed on the seabed, and more specifically to a coupling plate for such nodes.
Description of the Related Art
Marine seismic data acquisition and processing generates a profile (image) of a geophysical structure under the seafloor. Reflection seismology is a method of geophysical exploration to determine the properties of the Earth's subsurface, which is especially helpful in determining an accurate location of oil and gas reservoirs or any targeted features. Marine reflection seismology is based on using a controlled source of energy (typically acoustic energy) that sends the energy through seawater and subsurface geologic formations. The transmitted acoustic energy propagates downwardly through the subsurface as acoustic waves, also referred to as seismic waves or signals. By measuring the time it takes for the reflections or refractions to come back to seismic receivers (also known as seismic data recorders or nodes), it is possible to evaluate the depth of features causing such reflections. These features may be associated with subterranean hydrocarbon deposits or other geological structures of interest.
In general, either ocean bottom cables (OBC) or ocean bottom nodes (OBN) are placed on the seabed. For OBC systems, a cable is placed on the seabed by a surface vessel and may include a large number of seismic sensors, typically connected every 25 or 50 meters into the cable. The cable provides support to the sensors, and acts as a transmission medium for power to the sensors and data received from the sensors. One such commercial system is offered by Sercel under the name SeaRay®. Regarding OBN systems, and as compared to seismic streamers and OBC systems, OBN systems have nodes that are discrete, autonomous units (no direct connection to other nodes or to the marine vessel) where data is stored and recorded during a seismic survey. One such OBN system is offered by the Applicant under the name Trilobit®. For OBN systems, seismic data recorders are placed directly on the ocean bottom by a variety of mechanisms, including by the use of one or more of Autonomous Underwater Vehicles (AUVs), Remotely Operated Vehicles (ROVs), by dropping or diving from a surface or subsurface vessel, or by attaching autonomous nodes to a cable that is deployed behind a marine vessel.
Autonomous ocean bottom nodes are independent seismometers, and in a typical application they are self-contained units comprising a housing, frame, skeleton, or shell that includes various internal components such as geophone and hydrophone sensors, a data recording unit, a reference clock for time synchronization, and a power source. The power sources are typically battery-powered, and in some instances the batteries are rechargeable. In operation, the nodes remain on the seafloor for an extended period of time. Once the data recorders are retrieved, the data is downloaded and batteries may be replaced or recharged in preparation of the next deployment. Various designs of ocean bottom autonomous nodes are well known in the art. Prior autonomous nodes include spherical shaped nodes, cylindrical shaped nodes, and disk shaped nodes. Other prior art systems include a deployment rope/cable with integral node casings or housings for receiving autonomous seismic nodes or data recorders. Some of these devices and related methods are described in more detail in the following patents, incorporated herein by reference: U.S. Pat. Nos. 6,024,344; 7,310,287; 7,675,821; 7,646,670; 7,883,292; 8,427,900; and 8,675,446.
A conventional seismic node, whether placed on the seabed on its own with a navigation system, by an ROV, or by a cable, may include a base or lower plate that comprises radial grooves to help channel water during deployment and coupling to the seabed. For example, Applicant's Trilobit® node, described in U.S. Pat. No. 7,646,670, and incorporated herein by reference, illustrates a node with a conventional base plate. FIG. 2 from that patent is reproduced in the present application as FIG. 1A. The base plate may have grooves and/or holes on the underside of the plate to allow water and/or liquid mud to be expelled during deployment in water. Another node illustrating such radial designs is illustrated in U.S. Pat. No. 7,286,442, incorporated herein by reference. FIG. 4 from that patent is reproduced in the present application as FIG. 1B and shows ridges or grooves in the plate to enhance coupling to the ocean floor. Still another node illustrating such a grooved profile on the bottom plate is the KUM K/MT 210 seismometer, and similar to the other nodes discussed above, has radial grooves extending from the center of the plate.
The existing techniques for channeling water and/or liquid mud from the node as it couples to the seabed suffer from many disadvantages. With conventional plates, the seawater rushes out of the grooves at a high velocity from the center of the plate towards the circumference. As the gap distance between the bottom plate and the seabed approaches zero, the radial flow speed of exiting water may increase exponentially. If the node descent speed is sufficiently high, the radial flow may cause cavitation and temporary shear thinning of the seabed sediments, and thus, distort the effective acoustic coupling to the seabed.
A need exists for an improved node design for coupling to the seabed, and in particular one that allows for a more efficient and effective movement of water from the bottom of the node and/or the seabed coupling location.